Hello Friends, Is it possible to deduce the maximum no load speed of a brushless dc motor by the back EMF constant 'Ke' or from the motor velocity constant 'Kv'?
I found this article that describes how to find the maximum speed and tried following the steps:

https://things-in-motion.blogspot.com/2019/05/understanding-bldc-pmsm-electric-motors.html https://en.wikipedia.org/wiki/Motor_constants/

I have a motor with the maximum speed not mentioned in the specifications. A snap shot of the specifications are given below:
View attachment 329268
Here Ke is mentioned as 4.64 Vp/KRPM., converting it into the motor velocity constant (1000/4.64) , this comes to 216.4 RPM/V.
If the input supply voltage is 30 v (Vdc in the diagrm below), does this mean that the maximum speed the motor can theoretically operate is 30* 216.4 = 6492 RPM?
In some TI websites and application notes ,they mention that the input DC should be multiplied by 1/sqrt(3) first! (30/sqrt(3)) * 216.4 = 3750 RPM.
View attachment 329270



I am confused as to which is the correct equation?

The end goal here is to try and establish an equation that tells me the minimum Vdc value that is needed to run at rated speed.

 

I would have a tendency to say no, because the maximum no load speed is dependent on things besides Ke and/or Kv. IMHO your best bet will be to determine that empirically. We know for example that the torque is dependent on the current, but how much current can the windings handle. The quality of the bearings may have an effect as will the inertia of the rotor. There is more than a single factor at work here and your quest is not likely to yield the expected results.

 

Even if you know the maximum no load speed, is that really useful? As soon as you put the motor to practical use it will be driving a load.

 

Hi Papa/Alec

Even if you know the maximum no load speed, is that really useful? As soon as you put the motor to practical use it will be driving a load.

Agreed , but to do any further useful analysis of the motor drive system , I first need to know the maximum speed and then see how much it actually goes to , for the given Input DC, then later with addition of load (useful work) how much does the speed go to.

 

Hi Papa/Alec

Agreed , but to do any further useful analysis of the motor drive system , I first need to know the maximum speed and then see how much it actually goes to , for the given Input DC, then later with addition of load (useful work) how much does the speed go to.
View attachment 329522

Approach it any way you like but let us know what you find.